This invention relates to a catalyst which is useful for oxidation reactions. In particular, the invention relates to a catalyst which is efficient in converting alkanes, alkenes, or alcohols to unsaturated aldehydes and acids, and a process for preparing unsaturated aldehydes and acids using the catalyst.
Unsaturated aldehydes and acids are important commercial chemicals. Of particular importance is (meth)acrylic acid. The highly reactive double bond and acid function of (meth)acrylic acid makes it especially suitable as a monomer which may be polymerized alone or with other monomers to produce commercially important polymers. These unsaturated acids are also useful as a starting material for esterification to produce commercially important (meth)acrylate esters. Materials derived from (meth)acrylic acid or esters of (meth)acrylic acids are useful as plastic sheets and parts, paints and other coatings, adhesives, caulks, sealants, plastic additives, and detergents as well as other applications.
The production of unsaturated acids by oxidation of an olefin is well known in the art. Acrylic acid, for instance, may be commercially manufactured by the gas phase oxidation of propylene. It is also known that unsaturated carboxylic acids may also be prepared by oxidation of alkanes. For instance, acrylic acid may be prepared by the oxidation of propane. Such a process is especially desirable because alkanes generally have a lower cost than olefins. For example, at the time of filing this application propylene costs approximately three times more than propane. A suitable economic process for the oxidation of alkanes, as well as from oxidation of starting materials, to unsaturated aldehydes which is commercially viable has yet to be achieved.
There is continuing research in the area of new catalysts and starting materials for the production of (meth)acrylic acid and (meth)acrolein. This research generally is directed at reducing the cost of raw materials or increasing the yield of the oxidation process.
One impediment for the production of a commercially viable process for the catalytic oxidation of an alkane to an unsaturated acid is the identification of a catalyst having adequate conversion and suitable selectivity, thereby providing sufficient yield of the unsaturated acid end-product. U.S. Pat. No. 5,380,933 discloses a method for preparing a catalyst useful in the gas phase oxidation of an alkane to an unsaturated carboxylic acid. In the disclosed method, a catalyst was prepared by combining ammonium metavanadate, telluric acid and ammonium paramolybdate to obtain a uniform aqueous solution. To this solution was added ammonium niobium oxalate to obtain a slurry. The water was removed from the slurry to obtain a solid catalyst precursor. The solid catalyst precursor was molded into a tablet, sieved to a desired particle size and then calcined at 600xc2x0 C. under a nitrogen stream to obtain the desired catalyst.
Co-pending U.S. patent application Ser. No. 09/316,007 disclosed a process for preparing a catalyst for catalyzing an alkane into an unsaturated aldehyde or carboxylic acid wherein phase segregation was minimized and improvement in selectivity, conversion, and yield were achieved.
Despite the disclosure of the references, there is a continuing need for new catalysts and improved processes for the production of (meth)acrylic acid and/or (meth)acrolein.
In one aspect of the present invention, there is provided a catalyst having the formula:
AaMmNnXxOo 
wherein 0.25 less than a less than 0.98, 0.003 less than m less than 0.5, 0.003 less than n less than 0.5, 0.003 less than x less than 0.5, and o is dependent on the oxidation state of the other elements, and A is at least one of Mo, W, Fe, Nb, Ta, Zr, and Ru; M is at least one of V, Ce, and Cr; N is at least one of Te, Bi, Sb, and Se; and X is at least one of Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pd, Pt, Sb, Bi, B, In, and Ce; wherein the catalyst exhibits at least two crystal phases, one phase including major x-ray diffraction peaks at 22.1, 28.2, 36.2, 45.2, 50.5, 54.2, 55.4, and 58.5, and a second phase including major x-ray diffraction peaks at 22.1, 27.2, 35.3, 45.2, and 51.1.
In a second aspect of the present invention, there is provided a process for preparing unsaturated aldehydes and acids including subjecting an alkane to catalytic oxidation in the presence of a catalyst having the formula
AaMmNnXxOo 
wherein 0.25 less than a less than 0.98, 0.003 less than m less than 0.5, 0.003 less than n less than 0.5, 0.003 less than x less than 0.5, and o is dependent on the oxidation state of the other elements, and A is at least one of Mo, W, Fe, Nb, Ta, Zr, and Ru; M is at least one of V, Ce, and Cr; N is at least one of Te, Bi, Sb, and Se; and X is at least one of Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pd, Pt, Sb, Bi, B, In, and Ce; wherein the catalyst exhibits at least two crystal phases, one phase including major x-ray diffraction peaks at 22.1, 28.2, 36.2, 45.2, 50.5, 54.2, 55.4, and 58.5, and a second phase including major x-ray diffraction peaks at 22.1, 27.2, 35.3, 45.2, and 51.1.
In a third aspect, the present invention provides a process for preparing unsaturated aldehydes and acids including subjecting a compound selected from propylene, acrolein, and isopropanol to catalytic oxidation in the presence of a catalyst having the formula:
AaMmNnXxOo 
wherein 0.25 less than a less than 0.98, 0.003 less than m less than 0.5, 0.003 less than n less than 0.5, 0.003 less than x less than 0.5, and o is dependent on the oxidation state of the other elements, and A is at least one of Mo, W, Fe, Nb, Ta, Zr, and Ru; M is at least one of V, Ce, and Cr; N is at least one of Te, Bi, Sb, and Se; and X is at least one of Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pd, Pt, Sb, Bi, B, In, and Ce; wherein the catalyst exhibits at least two crystal phases, one phase including major x-ray diffraction peaks at 22.1, 28.2, 36.2, 45.2, 50.5, 54.2, 55.4, and 58.5, and a second phase including major x-ray diffraction peaks at 22.1, 27.2, 35.3, 45.2, and 51.1.
FIG. 1 depicts the x-ray diffraction (XRD) spectra of major XRD peaks for catalysts 1-7.
As used herein, the expression xe2x80x9c(meth)acrylic acidxe2x80x9d is intended to include both methacrylic acid and acrylic acid within its scope. In a like manner, the expression xe2x80x9c(meth)acrylatesxe2x80x9d is intended to include both methacrylates and acrylates within its scope and the expression xe2x80x9c(meth)acroleinxe2x80x9d is intended to include both acrolein and methacrolein within its scope.
As used herein the terminology xe2x80x9c(C3-C8) alkanexe2x80x9d means a straight chain or branched chain alkane having from 3 to 8 carbon atoms per alkane molecule.
As used herein the term xe2x80x9cmixturexe2x80x9d is meant to include within its scope all forms of mixtures including, but not limited to, simple mixtures as well as blends, alloys, etc.
For purposes of this application xe2x80x9c% conversionxe2x80x9d is equal to (moles of consumed alkane/moles of supplied alkane)xc3x97100; xe2x80x9c% selectivityxe2x80x9d is equal to (moles of formed desired unsaturated carboxylic acid or aldehyde/moles of consumed alkane)xc3x97100; and xe2x80x9c% yieldxe2x80x9d is equal to (moles of formed desired unsaturated carboxylic acid or aldehyde/moles of supplied alkane)xc3x97(carbon number of formed desired unsaturated carboxylic acid or aldehyde/carbon number of the supplied alkane)xc3x97100.
For purposes of this application by xe2x80x9csolutionxe2x80x9d is meant that greater than 95 percent of metal solid added to a solvent is dissolved. It is to be understood that the greater the amount of metal solid not initially in solution, the poorer the performance of the catalyst derived therefrom will be.
As recited above, a catalyst having at least two specific crystal phases is disclosed. The two crystal phases may be obtained either through a specific method of preparation of the catalyst or through varying the composition of the catalyst.
In a first step of the method of preparation of the catalyst, a solution is formed by admixing metal compounds, at least one of which contains oxygen, and at least one solvent in appropriate amounts to form the solution. Generally, the metal compounds contain elements A, M, N, X, and O. In one embodiment, A is at least one of Mo, W, Fe, Nb, Ta, Zr, and Ru; M is at least one of V, Ce, and Cr; N is at least one of Te, Bi, Sb, and Se; and X is at least one of Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pd, Pt, Sb, Bi, B, In, and Ce. In a preferred embodiment, A is at least one of Mo and W; M is at least one of V, Ce, and Cr; N is at least one of Te, Bi, and Sb; and X is at least one of Nb, Ta, and Zr. In a more preferred embodiment, A is Mo, M is V, N is Te and X is Nb.
Suitable solvents include water, alcohols including, but not limited to, methanol, ethanol, propanol, and diols etc, as well as other polar solvents known in the art. Generally, water is preferred. The water is any water suitable for use in chemical synthesis including, without limitation, distilled water and deionized water. The amount of water present is that amount sufficient to keep the elements substantially in solution long enough to avoid or minimize compositional and/or phase segregation during the preparation steps. Accordingly, the amount of water will vary according to the amounts and solubility of materials combined. However, as stated above the amount of water must be sufficient to insure an aqueous solution is formed and not a slurry at the time of mixing.
Once the aqueous solution is formed, the water is removed by a combination of any suitable methods known in the art to form a catalyst precursor. Such methods include, without limitation, vacuum drying, freeze drying, spray drying, rotary evaporation, and air drying. Vacuum drying is generally performed at pressures ranging from 10 to 500 mm/Hg. Freeze drying typically entails freezing the solution, using for instance liquid nitrogen, and drying the frozen solution under vacuum. Spray drying is generally performed under an inert atmosphere such as nitrogen, argon, or air with an inlet temperature ranging from 125xc2x0 C. to 200xc2x0 C. and an outlet temperature ranging from 75xc2x0 C. to 150xc2x0 C. Rotary evaporation is generally performed at a bath temperature of from 25xc2x0 C. to 90xc2x0 C. and a pressure of from 10 mm/Hg to 760 mm/Hg, preferably at a bath temperature of from 40xc2x0 C. to 90xc2x0 C. and a pressure from 10 mm/Hg to 350 mm/Hg, more preferably from 40xc2x0 C. to 60xc2x0 C. and a pressure of from 10 mm/Hg to 40 mm/Hg. Air drying may be occur at temperatures ranging from 10xc2x0 C. to 90xc2x0 C. Rotary evaporation or air drying are generally preferred.
Once obtained, the catalyst precursor is calcined under an inert atmosphere. The inert atmosphere may be any material which is substantially inert, i.e., does not react or interact with, the catalyst precursor. Suitable examples include, without limitation, nitrogen, argon, xenon, helium or mixtures thereof. Preferably, the inert atmosphere is argon or nitrogen, more preferably argon. The inert atmosphere may flow over the surface of the catalyst precursor or may not flow (a static environment). It is important to understand that by non-flow atmosphere is meant that the inert gas is not allowed to flow over the surface of the catalyst precursor. It is preferred that the inert atmosphere not flow over the surface of the catalyst precursor. However, when the inert atmosphere does flow over the surface of the catalyst precursor, the flow rate can vary over a wide range, for example, at a space velocity from 1 to 500 hrxe2x88x921.
The calcination is typically done at a temperature of from 350xc2x0 C. to 850xc2x0 C., preferably from 400xc2x0 C. to 700xc2x0 C., more preferably from 500xc2x0 C. to 640xc2x0 C. The calcination is typically performed for an amount of time suitable to form the catalyst. In one embodiment, the calcination is performed from 0.5 to 30 hours, preferably from 1 to 25 hours and more preferably from 1 to 15 hours.
With calcination a catalyst is formed having the formula
AaMmNnXxOo 
wherein A, M, N, and X are as described above. Molar ratios, a, m, n, and x are typically, from 0.25 less than a less than 0.98, 0.003 less than m less than 0.5, 0.003 less than n less than 0.5, and 0.003 less than x less than 0.5; preferably 0.35 less than a less than 0.87, 0.045 less than m less than 0.37, 0.020 less than n less than 0.27, and 0.005 less than x less than 0.35. The catalyst prepared will exhibit at least two crystal phases, one phase including major x-ray diffraction peaks at 22.1, 28.2, 36.2, 45.2, 50.5, 54.2, 55.4, and 58.5, and a second phase including major x-ray diffraction peaks at 22.1, 27.2, 35.3, 45.2, and 51.1. The second phase further includes x-ray diffraction peaks at 7.9, 9.1, and 29.2. Such major peaks and peaks are more completely defined in Table 1 (XRD Peaks of Phase-A) and Table 2 (XRD Peaks of Phase-B) following.
The molar ratio, xe2x80x9coxe2x80x9d i.e., the amount of oxygen (O) present, is dependent on the oxidation state of the other elements in the catalyst. However, typically xe2x80x9coxe2x80x9d is from 3 to 4.7, based on the other elements present in the catalyst.
If the composition of the catalyst is varied outside of the ranges defined above, the catalyst will not exhibit both x-ray diffraction phases.
The catalyst of this invention may be used as a solid catalyst alone or may be utilized with a suitable support such as, without limitation, silica, alumina, titania, aluminosilicate, diatomaceous earth, or zirconia. The shape of the catalyst can be any suitable shape and will depend upon the particular application of the catalyst. In a like manner, the particle size of the catalyst may be any suitable particle size depending on the particular use of the catalyst.
In the second aspect of the present invention, there is provided a process for preparing unsaturated aldehydes and acids including subjecting an alkane to catalytic oxidation in the presence of the catalyst described above.
The starting materials for this process are generally an alkane gas or gases and at least one oxygen-containing gas. It is preferred that the starting materials also include steam. Accordingly, a starting material gas is supplied to the system which includes a gas mixture of at least one alkane and steam. The at least one oxygen-containing gas may be included in this mixture or be supplied separately. Furthermore, a diluting gas such as an inert gas including, without limitation, nitrogen, argon, helium, steam, or carbon dioxide may also be included. The diluting gas may be used to dilute the starting material and/or to adjust the space velocity, the oxygen partial pressure, and the steam partial pressure.
Suitable molar ratios of the alkane/oxygen/diluting gas/water in the starting material gas mixture are known in the art as well as the feed ratio of alkane/air/steam. For instance suitable ranges are disclosed in U.S. Pat. No. 5,380,933.
The starting material alkane is generally any alkane suitable for gas phase oxidation into an unsaturated aldehyde or acid. Generally, the alkane is a C3-C8 alkane, preferably propane, isobutane or n-butane, more preferably propane or isobutane, most preferably propane. Furthermore, in another embodiment the alkane may be a mixture of alkanes including C3-C8 alkanes as well as lower alkanes such as methane and ethane.
The at least one oxygen-containing gas used may be pure oxygen gas, an oxygen containing gas such as air, an oxygen enriched gas, or a mixture thereof.
In a preferred embodiment, the starting material is a gas mixture of propane, air, and steam. The starting gas mixture is subjected to catalytic oxidation in the presence of the catalyst of the present invention. The catalyst may be in a fluidized bed or a fixed bed reactor. The reaction is generally conducted under atmospheric pressure, but may be conducted under elevated or reduced pressure. The reaction temperature is generally from 200xc2x0 C. to 550xc2x0 C., preferably 300xc2x0 C. to 480xc2x0 C., more preferably 350xc2x0 C. to 440xc2x0 C. The gas space velocity is generally 100 to 10,000 hrxe2x88x921, preferably 300 to 6,000 hrxe2x88x921, more preferably 300 to 3,000 hrxe2x88x921.
Also, in the method of the present invention it is to be understood that an unsaturated aldehyde may also be formed. For instance when propane is the starting alkane, acrolein may be formed and when isobutane is the starting alkane, methacrolein may be formed.
In the third aspect of the present invention, there is provided a process for preparing unsaturated aldehydes and acids including subjecting a compound selected from propylene, acrolein, and isopropanol to catalytic oxidation in the presence of the catalyst described above. The process is run in the same manner as described above for the conversion of alkanes to unsaturated aldehydes or acids, except propylene, acrolein, or isopropanol is substituted for the alkane. Also, the reaction temperature is generally from 150xc2x0 C. to 500xc2x0 C. For propylene and isopropanol the reaction temperature is preferably 250xc2x0 C. to 400xc2x0 C., and for acrolein preferably 180xc2x0 C. to 350xc2x0 C. The gas space velocity is generally 100 to 10,000 hrxe2x88x921, preferably 300 to 6,000 hrxe2x88x921.
Abbreviations used throughout this application are:
The following examples illustrate the process of the present invention. Based on the amount of starting material used, if there was no compositional segregation, or there was no loss of certain elements during the preparation steps, all of the catalyst samples prepared as follows should have an empirical formula of Mo1V0.3Te0.23Nb0.08-0.12On where n is determined by the oxidation state of the other elements. The solutions or slurries containing the desired metal elements were prepared by heating the appropriate compounds in water at a temperature ranging from 25xc2x0 C. to 95xc2x0 C. When necessary, the solutions or slurries were cooled to temperatures ranging from 25xc2x0 C. to 60xc2x0 C. The water was then removed from the solutions or slurries by the appropriate drying method at pressures ranging from 760 mm/Hg to 10 mm/Hg.